1. Field of the Invention
The present invention relates generally to power converters, and more particularly to switchable power converters for multiple level input line power factor correction.
2. Background of the Invention
FIG. 1 illustrates a conventional power converter circuit operating as a high frequency electronic ballast for multiple gas discharge lamps 190. Referring to FIG. 1, the power converter circuit basically comprises two stages. The front end is a boost converter 100 for universal line power factor correction and universal line voltage regulation. The boost converter 100 is primarily comprised of a power switch 102, inductor 104, diode 106, and DC bus capacitor 108.
The back end is a typical voltage-fed half-bridge inverter 140 loaded with the lamps 190 through a resonant tank circuit comprised of a capacitor 152 and inductor 154, along with any magnetizing inductance generated by output transformer 156. The half-bridge inverter is primarily comprised of power switches 148 and 150.
The boost converter of FIG. 1 is ideal for providing a DC bus voltage 112 of 450 VDC (across capacitor 108) for input voltages of 120V/277V AC. The relatively high DC output voltage level is due to the fact that the intrinsic topology of the boost converter requires the DC bus voltage 112 to be greater than the peak value of the input line voltage.
However, some applications require a lower DC bus voltage, for example 225V DC. In those applications a flyback converter is better suited, since the flyback converter is capable of generating the relatively low DC bus voltage of 225V DC from an input voltage of 120V/277VAC. The flyback converter, however, has several drawbacks, including higher component stresses, lower overall efficiency, larger component sizes, and severely large electromagnetic interference (EMI) conditions.
Alternatively, a single-ended primary inductance converter (SEPIC) may be employed. The SEPIC is capable of producing an intermediate DC output voltage, such as 225V DC. While the SEPIC shares some of the drawbacks of the flyback converter, such as higher component stress, lower overall efficiency and larger size, the SEPIC enjoys improved EMI conditions. This is because the SEPIC input section is similar to the boost converter input section.
It is a characteristic of both the flyback and SEPIC converters that the highest losses occur at the lowest input line voltage and the highest voltage stresses occur at the highest input line voltage over a universal input line voltage range. Among the flyback, SEPIC and boost converters, the boost converter exhibits the highest efficiency and lowest voltage stresses. However, as discussed above, the boost converter is only viable for use at lower input line voltages in the case of 225V DC bus voltage specifications.
A switchable power converter is therefore needed that advantageously switches between a boost converter circuit topology, for low input line voltages, and either a flyback or SEPIC converter circuit topology, for high input line voltages, to provide an intermediate DC output voltage level, such as 225V DC, over a range of input line voltage levels.
It is therefore an object of the present invention to provide a switchable power converter.
It is another object of the present invention to provide a switchable power converter having improved efficiency and reduced stresses over a range of input line voltages.
To achieve the above objects, a switchable power converter in accordance with the present invention includes an input section that receives an AC input voltage and rectifies the AC input voltage and a switchable converter section operative to receive the rectified AC input voltage and convert the rectified AC input voltage to an intermediate DC output voltage. The switchable converter section includes at least one configuration switch operative to switch the switchable converter section between a boost converter topology, for low input line voltages, and either a flyback or SEPIC converter circuit topology, for high input line voltages. The configuration switch may be a relay based mechanical switch or a solid state switch, for example.